The 3.3-V LDO is enabled or disabled by setting or clearing VUSBEN, respectively. Even if enabled, if the voltage on VBUS is detected to be low or nonexistent, the LDO is suspended. No additional current is consumed while the LDO is suspended. When VBUS rises above the USB power brownout level, the LDO reference and low voltage detection become enabled. When VBUS rises further above the launch voltage VLAUNCH, the LDO module becomes enabled (see Figure 42-3). See device-specific data sheet for value of VLAUNCH.
The 1.8-V LDO can be enabled or disabled by setting SLDOEN accordingly. By default, the 1.8-V LDO is controlled automatically according to whether power is available on VBUS. This auto-enable feature is controlled by SLDOAON. In this case, that the SLDOEN bit does not reflect the state of the 1.8-V LDO. If the user wishes to know the state while using the auto- enable feature, the USBBGVBV bit in
USBPWRCTL can be read. In addition, to disable the 1.8-V LDO, SLDOAON must be cleared along with SLDOEN. If providing VUSB from an external source, rather than through the integrated 3.3-V LDO, keep in mind that if 5 V is not present on VBUS, the 1.8-V LDO is not automatically enabled. In this situation, either VBUS much be attached to USB bus power, or the SLDOAON bit must be cleared and SLDOEN set.
It is required that power from the USB cable's VBUS be directed through a Schottky diode prior to entering the VBUS terminal. This prevents current from draining into the cable's VBUS from the LDO input, allowing the MSP430 to tolerate a suspended or unpowered USB cable that remains electrically connected.
The VBONIFG flag can be used to indicate that the voltage on VBUS has risen above the launch voltage. In addition to the VBONIFG being set, an interrupt is also generated when VBONIE = 1. Similarly, the VBOFFIFG flag can be used to indicate that the voltage on VBUS has fallen below the launch voltage. In addition to the VBOFFIFG being set, an interrupt is also generated when VBOFFIE = 1. The USBBGVBV bit can also be polled to indicate the level of VBUS; that is, above or below the launch voltage.